A liquid crystal display device is thin and light, and therefore, has been widely used as a display for a personal digital assistant. A passive liquid crystal device which does not emit a light itself and expresses a display by changing the transmissivity and can be driven with several volts of effective voltage. Accordingly, when the liquid crystal display device is used as a reflective one which comprises a reflector provided at the lower side of the liquid crystal display device and expresses a display by reflecting the outside light, a display device with an extremely low consumption of electricity can be provided.
A conventional reflective color liquid crystal display device comprises basically a liquid crystal cell having a color filter and a pair of polarization films by which the liquid crystal cell is sandwiched. In this case, the color filter is provided on one of substrates of the liquid crystal cell, and a transparent electrode is formed on the color filter provided on the substrate of the liquid crystal cell. A voltage is applied to the liquid crystal cell to change the alignment of the liquid crystal molecules. Accordingly, the transmissivity of each color filter is changed so as to express a color display.
The transmissivity of one polarization film is 45% at most, as a whole. In this case, the transmissivity of a polarization component parallel to an adsorption axis of the polarization film is about 0% and the transmissivity of a polarization component perpendicular to the adsorption axis of the polarization film is about 90%. Accordingly, in a reflective liquid crystal display device comprising two polarization films, when the outside light is incident and reflected by a reflector and goes outside, the light passes through the polarization film four times. As a result, when adsorption by the color filter is not taken into account, the reflectance of the outside light is obtained by the following formula; EQU 0.9.sup.4.times.50%=32.8%
Accordingly, when the color filter of the conventional reflective color liquid crystal display device is removed and the device is used as a black and white panel, in the same way, the reflectance reaches 33% at most.
In order to brighten a light display, several reflective liquid crystal display devices comprising only one polarization film provided on the upper side of a liquid crystal cell that is sandwiched by the polarization film and a reflector have already been proposed (refer to Japanese Laid-Open Patent Publication No. 7-146469 and No. 7-84252). In these proposed reflective liquid crystal display devices, the light passes through the polarization film only twice. When adsorption by a color filter is not taken into account, the reflectance of the outside light is obtained by the following formula. EQU 0.9.sup.2.times.50%=40.5%
Therefore, these proposed reflective liquid crystal display devices can be expected to increase the reflectance by at most about 23.5% in comparison with that of the liquid crystal display device comprising two polarization films.
A reflective color liquid crystal display device in which a color display is expressed by using the birefringence of a twist orientation nematic liquid crystal layer and a polarization film, without using a color filter, has been disclosed in Japanese Laid-Open Patent Publication No.6-308481. A color liquid crystal display device in which the birefringence of liquid crystal and phase difference film is used has been disclosed in Japanese Laid-Open Patent Publication No.6-175125 and No.6-301006.
However, a reflective liquid crystal device comprising two polarization films can not have sufficient reflectance to obtain sufficient brightness when a color display is expressed by using the additional color filter.
Further, a reflective liquid crystal display device comprising one polarization film expresses a color display using a color filter and the brightness is maintained by increasing the reflectance. In the above-mentioned conventional constitution, it is difficult to express a black and white display in achromatic color, and it is especially difficult to express a black display in achromatic color with low reflectance.
In the reflective color liquid crystal display device in which a colored display is expressed by using the birefringence of twist alignment nematic liquid crystal layer and a polarization film, without using a color filter, or the color liquid crystal display device in which the birefringence of liquid crystal and phase difference film is used, since a color filter is not provided, the efficient reflectance to obtain the practical brightness can be ensured even if two polarization films are used. However, in the above-mentioned color liquid crystal display devices, since a color display is expressed by using the birefringence, on the principle, it is difficult to express a multi-grade and multi-color display such as a 16 grade 4096 color display or a 64 grade full-color display. Further, the above-mentioned color liquid crystal display devices have other problems such as the range of color purity and color reproduction is also being narrow.
In a reflective liquid crystal display device with black and white mode comprising two polarization films, a white display with high reflectance can't be obtained, either.